Thin film heaters typically comprise a growth or deposition of a thin film of electrically conductive resistive material on an electrically insulating support substrate, e.g., glass, quartz, glass ceramic, alumina, etc. Alternatively, the thin film material can be deposited upon a substrate that is electrically conductive, e.g., stainless steel, if the deposition surface of the substrate is first treated with a dielectric coating, e.g., DuPont™ material part number 3500, or Electro Science Laboratories material, part number 4914. Other dielectric coating materials may be used, if desired.
In a thin film heater, when a source of voltage is coupled across the thin film resistive material, the resultant electrical current causes the thin film to generate heat. However, heat is not generated uniformly across the surface of the thin film heater, apparently due to varying current densities within the thin film material. Heat variation can be large near the perimeter edges of the thin film material or near heat sinks, where cooling effects are predominate and so-called edge thermal loss occurs. But some applications require that thin film heaters generate heat substantially uniformly so as to maintain a target set point temperature accurate to within about ±5° C. across the heater surface, including surface regions near the heater edges. Unfortunately achieving this goal is not readily accomplished in the prior art.
FIG. 1A depicts a conventional prior art thin film heater 10 in which the upper surface of a supporting substrate 10 (shown cross-hatched) is covered with a growth or deposition of thin film electrically conductive material 30.
Electrically conductive buss bar structures 40 are placed typically at opposite edges of the thin film material and will be connected by wires 50 to a source of voltage (Vs) 60. Buss bar structures in FIG. 1A (and in the other figures) can have a width of about 0.040″ to about 0.375″. Typically magnitudes of voltage Vs are about 3 V to about 240 V.
When electrical power Vs is connected, electrical current flows across thin film material 30, generating heat, much of which is transferred to the underlying substrate material 20. The efficiency of a thin film heater is a direct function of the substrate material type and mass. Efficiency is essentially the ability of a thin film heater to convert a given amount of energy to heat, to achieve a given rate of thermal increase, and to distribute thermal energy over the entire mass of the thin film heater. Conventional thin film heaters can achieve power densities exceeding 100 watts/square inch, and can attain temperatures exceeding 450° C. (840° F.). However, distribution of heat over the surface of the thin film heater can be uneven due to the effects of the geometry of the unit itself.
Substrate 10 preferably has a smooth upper surface and is made from material that includes, without limitation, glass, quartz, ceramic (alumina), aluminum nitride, silicon carbide, stainless steel, porcelainized steel. These or other materials can also be used, which materials can be in tubular, disk, block or sheet form. Such material types provide an electrically insulating surface upon which the layer of thin film material 20 will be applied. Further, such substrate materials can sustain the high temperatures desired for a heater, and are physically self-supporting. It is understood that where the material is a metal, e.g., stainless steel, the surfaces including the upper surface will be electrically insulating for example by virtue of a dielectric layer deposited on the substrate.
FIG. 1A depicts the various temperatures (in ° C.) attained at different locations on thin film material 20 for thin film heater 10, for which the desired and intended thermal set point was 150° C. It will be appreciated that there is substantial non-uniformity in the distribution of heating across the surface of heater 10. Heat variations can be especially troublesome at the peripheral edges and corners of thin film heater 10. In many heater structures, thin film material 30 substantially covers the entire upper surface of substrate 20. In other structures, one or more margins 70 may be required, which is to say, thin film material 30 will not completely cover all of the underlying surface of substrate 20 in the margin region. For example, thin film heaters that will be mounted or retained in a frame-like holder may require the presence of margins 70, and thus the exclusion of overlying thin film material in these regions. Understandably the presence of such margins can further complete the challenge of trying to generate heat uniformly across the heater surface. Such thermal edge losses can also occur in flat, round, tubular and other shaped thin film heater structures where there is a substantial surface area.
Heater 10 in FIG. 1A (as well as in FIG. 1B) has a thickness T of about 0.025″ and is about 1″×1″ in size, and for the thermal data shown in the figure has a thick ceramic tin oxide substrate 20. As noted, the desired thermal set point for the thin film heater 10 in FIG. 1A was 150° C. But as shown by the temperature values in FIG. 1A, the actual temperature attained at different regions of the heater deviate from this set point target by several ° C. The maximum temperature attained is 153° C., the lowest temperature is 147° C., and the thermal uniformity is ±3° C. Note for example that although the heater configuration is essentially symmetrical, e.g., square, temperature at the upper left corner of the heater missed the set point temperature by 3° C., while temperature at the lower left corner exceeded the set point temperature by 2° C. In many applications, such thermal non-uniformities may not be acceptable.
Heater 10 in FIG. 1B is identical the heater shown in FIG. 1A except that a set point temperature of 350° C. was desired. Unfortunately a substantial amount of thermal non-uniformity is apparent, with the maximum temperature being 351° C., the lowest temperature being 331° C., with a thermal uniformity of only ±10° C.
Heater 10 in FIG. 1C was formed on a polished quartz substrate 20, and was about 3.07″×4.82″ in size, with a thickness T of about 0.125″. Note that length of the thin film material 30 was intentionally made shorter than the length of the underlying substrate 20 such that margins 70 were formed on the short sides of the rectangular structure. A set point temperature of 114° C. was desired. Point 80 in FIG. 1C represents the center of the thin film heater element 30, at which location the 114° C. set point temperature should exist. But as shown, even at point 80 the set point temperature was missed (by 1° C.), and actual temperature across the surface of thermal element 30 varied from about 84° C. to 115° C., with a thermal uniformity of only ±15.5° C.
FIG. 1D depicts thermal variation in a larger sized heater 10 that measured 16″×24″×T=0.157″, and had a glass ceramic material as substrate 20. A margin 70 of 1″ surrounded the thin film material 30. The thermal set point temperature was 150° C., but actual temperature across the surface of thermal element 30 ranged from 101° C. to 155° C., a variation of ±27° C. Note that even at the center point 80, the target set point temperature was not attained. In FIG. 1D, the “° C.” symbols were omitted to make the data shown more readable.
FIG. 1E is a top plan view of a somewhat narrow thin film heater 10 that measured 0.360″×10.625″×T=0.025″. Substrate 20 was alumina with a thin film tin oxide coating. The numbers above the figures are measured temperature values in ° C. (where the “° C.” symbol has been omitted due to space constraints). These temperatures were measured on the surface of the thin film heater element 30 at locations shown with a “dot”, generally adjacent the temperature value. The dashed lines traversing the narrow width of thin film heater 10 denote different heating regions. In this embodiment the target set point temperature was 180° C., yet temperatures ranged from 99° C. to 193° C.
FIG. 1F is a side view of a tubular, rather than flat, thin film heater 10 formed about a quartz substrate 20, with a tin oxide resistive layer 30. Margins 70 were formed as shown. Two columns of temperature data (with the “° C.” symbol omitted) denoted “TEMP A1” and “TEMP B1” are shown to the right of the heater. Column A1 data are temperature measured on the inside wall of the heater tube 20, and column B1 data are temperatures measured inside of a Teflon™ material tube inserted within the heater structure. (The temperature data were measured using a thin wire thermocouple.) The Vs power source 60 and power leads 50 are omitted from FIG. 1F for clarity. Resistance Rbb, measured between buss bar structures 40, was about 81.2 Ohms. Various dimensions in inches are shown to the left and above the heater structure. As shown, the outer diameter of the heater was about 0.125″, and the nominal target set point temperature was 95° C.
FIG. 1G is a side view of a somewhat similar tubular thin film heater 10, again formed with a quartz substrate 20 and a tin oxide resistive layer 30, and with margins 70. Thermal data for columns A1 and B1 represent temperatures (in ° C.) measured, respectively, on the inside wall of heater tube 20, and inside a Teflon™ material tube inserted within the heater structure. Resistance Rbb, measured between buss bar structures 40, was about 74.2 Ohms, and nominal target set point temperature was 95° C.
In reviewing the various prior art thin film heater embodiments shown in FIGS. 1A–1G, it is seen that in general the larger the surface area of a thin film heater, the more pronounced will be the thermal non-uniformity experienced by an object in contact with or in close proximity to the heated surface. In tube-shaped thin film heaters, FIGS. 1F and 1G, for example, or long narrow strip-shaped thin film heaters, pronounced loss of thermal energy is manifested at the ends of the heater structure.
Exemplary techniques for fabricating prior art thin film heaters are found in several U.S. patents. For example, U.S. Pat. No. 5,616,266 (1997) to Cooper discloses a cooking-type heater in which a thin film is formed on a ceramic-based layer atop a rigid metallic substrate, the goal being to attain 300° F. on an 18″×18″ surface with a power density of about 6.17 watts/in2, while consuming approximately 2 kW of electrical operating power. U.S. Pat. No. 6,376,816 (2002) to Cooper discloses a thin film heater useful to heat liquids. In Cooper '816, regions of thin film conducting material are molecularly bounded to outer surface regions of a tubular substrate to form the overall tubular heater. Neither of these exemplary two patents disclosed data regarding uniformity of heat distribution for the described thin film heaters.
Thus, there is a need for a method and structure by which thin film heaters can be fabricated so as to compensate for thermal edge loss. Fabrication of such thin film heaters preferably should use commercially available equipment, and the resultant heater should attain a target set point temperature with improved thermal uniformity over the heater surface.
The present invention provides such a thin film heater and methods for fabricating such thin film heaters.